Low-temperature fuel cell assembly and method for the operation thereof

ABSTRACT

Low-temperature fuel cell assembly consisting of a low-temperature fuel cell in combination with a sweeping gas circuit for at least the anode part thereof. The fuel gas is prepared by filtering via a semi-permeable wall and the yield from this process is increased by employing sweeping gas. This sweeping gas is obtained by producing it in the sweeping gas circuit. During operation some of the sweeping gas is continuously discharged and fresh sweeping gas is continuously produced. With this arrangement it is possible to use hydrogen as fuel gas. Sweeping gas is obtained by injecting air and combusting the oxygen contained therein with hydrogen, It is also possible to perform a methanisation reaction starting from hydrogen as fuel gas. by providing CO/CO 2 , as a result of which the sweeping gas is methane.

[0001] The present invention relates to a low-temperature fuel cellassembly comprising a fuel cell having a sweeping gas circuit for fuelgas, said sweeping gas circuit being connected to the inlet and outletof the anode of the fuel cell and supply means being provided for fuel,which is connected to said sweeping gas circuit via a physicalseparation device, means being arranged in the sweeping gas circuit forproducing sweeping gas therein.

[0002] A low-temperature fuel cell of this type can be an SPFC cell(solid polymer fuel cell), but it must be understood that this term isalso understood to cover any other low-temperature fuel cell, such as aphosphoric acid fuel cell. Low temperature is understood to be anoperating temperature of below 200° C. In the case of an SPFC cell, thefuel gas used is frequently hydrogen and hydrogen is obtained by areforming reaction, for example from methane. Admission to the fuelcircuit takes place through a semi-permeable wall or correspondingtechnique such as sorption, hydrogen atoms/molecules being allowed topass through, but not other gases. In order to optimise the passagethrough such a semi-permeable wall it is proposed in the prior art touse a sweeping gas in the fuel supply. This sweeping gas must preferablybe inert, is not itself consumed in the fuel cell and is largelyrecycled. It is for this reason that a sweeping gas circuit is present.As a result of the use of the sweeping gas the concentration of hydrogenor other fuel gas is lower and accelerated passage through thesemi-permeable wall or other filter will take place, whilst sufficienthydrogen is admitted for operation of the fuel cell.

[0003] An assembly of this type is disclosed in U.S. Pat. No. 5,976,724.In this assembly the anode gas is supplied via a semi-permeable membraneto a closed anode circuit. In this assembly water is used as sweepinggas. This sweeping gas is generated by evaporation of water present inthe circuit as a liquid, which is condensed before entry into the anode.

[0004] As far as the sweeping gas is concerned, such a circuit iscompletely closed. It has been found that poisoning of the sweeping gasoccurs during long-term use of such a fuel cell assembly, as a result ofwhich the efficiency of the fuel cell falls (dramatically).

[0005] The aim of the present invention is to avoid this disadvantage.

[0006] Said aim is achieved with an assembly as described above in thatthe means comprise chemical reactor means having an inlet for onecomponent of said chemical reaction and in that the sweeping gas circuitis provided with a sweeping gas discharge.

[0007] According to the present invention it is proposed to replace thesweeping gas discharged by sweeping gas that has been produced in thesweeping gas circuit by chemical means. That is to say, no separate(initial) supply of sweeping gas is required. As a result, such alow-temperature fuel cell assembly can be of appreciably simplerconstruction with corresponding consequences for the cost price thereofand the operation thereof, as a result of which the operating costs willfurther decrease.

[0008] The production of the sweeping gas takes place depending on thesweeping gas used.

[0009] It is noted out that PCT application WO 001/16426, which has notbeen published on the priority date of this application, discloses anassembly in which a fuel gas is fed to a sweeping gas circuit without aphysical separation device, such as a semi-permeable membrane. Carbonmonoxide is removed by a chemical reaction. All fuel present in thesweeping gas circuit is immediately combusted stoichiometrically and thewater liberated during this combustion is discharged. In the case of thepresent invention unconsumed anode fuel is preferably circulated so thatthis can still be usefully used at a later stage. Anode fuel gas isadditionally consumed only to make up the quantity of sweeping gas thathas been removed and removal of sweeping gas takes place for the reasonsmentioned above.

[0010] According to a first particular embodiment of the invention, air(optionally exclusively oxygen) is introduced into the sweeping gascircuit. At this point the fuel cell is still not in operation. Gas, inparticular hydrogen, originating from the reforming device or otherhydrogen generator reacts in a controlled manner with this air in aburner present in the sweeping gas circuit. During this reaction oxygenpresent in the air is converted to water vapour and carbon monoxidewhich is produced during this reaction and/or may be present isconverted to carbon dioxide. The mixture consisting of water vapour,carbon dioxide and nitrogen is found to function exceedingly well as asweeping gas. The cell is then brought into operation.

[0011] Because, for example, the selective effect of the semi-permeablemembrane or other filter in the reforming device is not complete, in themajority of cases it cannot be precluded that carbon monoxide or otherharmful substances are present in the fuel gas during the progressiveoperation of the fuel cell. Carbon monoxide is particularly harmful forthe anode. Any platinum present in the fuel cell is poisoned by carbonmonoxide.

[0012] According to the invention it is proposed to remove a smallamount of sweeping gas continually during operation of the fuel cell inorder to prevent accumulation of undesired gases. This amount iscompensated for by the continual introduction of a small amount of air.This amount of air reacts with the fuel gas (hydrogen). This canoptionally be carried out in the abovementioned burner. In this contextit is preferable that the introduction of air takes place upstream ofthe anode inlet. As a result of the presence of air, more particularlyoxygen, any carbon monoxide present can be converted to carbon dioxide,which is harmless for the fuel cell.

[0013] Instead of using a mixture of water vapour, CO₂ and nitrogen assweeping gas, it is possible to use methane as sweeping gas. Acombination with the gases described above is optionally possible.

[0014] Methane can be obtained in the sweeping gas circuit by convertingthe CO/CO₂ present therein or fed thereto to methane with the aid ofmethanising means. These methanising means make it possible to obtainmethane and water vapour from hydrogen and carbon monoxide/carbondioxide. With this embodiment there is a lesser need to remove sweepinggas continually during the process because any carbon monoxide presentin the fuel gas is immediately converted to methane in the methanisingmeans. Discharge may be needed in connection with other harmful gasesand to prevent too much methane being present in the circuit.

[0015] The construction according to the invention makes it possible toincrease the efficiency of fuel cell assemblies appreciably, that is tosay fuel cells in combination with reformers, because a sweeping gascircuit can be used. On the other hand, the construction is notparticularly complex because the sweeping gas is produced in the systemitself prior to start-up thereof and during operation of the fuel cell.

[0016] The invention will be explained in more detail below withreference to illustrative embodiments shown in the drawing. In thedrawing:

[0017]FIG. 1 shows a block diagram of a first embodiment of the assemblyaccording to the invention where air is used as sweeping gas; and

[0018]FIG. 2 shows an assembly according to FIG. 1, where methane isused as sweeping gas.

[0019] In FIG. 1 a solid polymer fuel cell is indicated by 1. This fuelcell consists of a cathode side 10 and an anode side 11 separated by anelectrolyte 20. In the context of the present invention only the anodeside 11 is of importance. This anode side has an inlet 6 and an outlet7. The fuel cell shown here is preferably operated on hydrogen. Hydrogenoriginates from a reforming device 30 to which, for example, natural gasor other gas is supplied. With this arrangement hydrogen is formed by areaction, but before admitting this hydrogen to the inlet 6 of the anodefiltering is necessary in order to separate off any undesired gasespresent. Separation takes place in receiving section 4 with asemi-permeable membrane 5. In order as far as possible to promote thetransfer of hydrogen through this semi-permeable membrane 5, theconcentration thereof on the permeate side 21 must be as low aspossible. This is achieved by the presence of a sweeping gas in asweeping gas circuit 2. The sweeping gas is enriched with hydrogen onthe permeate side 21 and then fed through the anode and here (some)hydrogen but no sweeping gas is consumed. On the one hand it is possibleby this means to supply sufficient hydrogen to the anode 11 of the fuelcell 1 but, on the other hand, the concentration when fed to thepermeate side 21 is so low that transfer of hydrogen through thesemi-permeable membrane 5 is promoted as far as possible.

[0020] An air feed is indicated by 8. An advantageous embodiment is onein which cathode discharge gas with a low oxygen content is used forthis, as a result of which less hydrogen is consumed and the systemefficiency increases. 9 indicates a burner. 3 is a pump for maintainingthe circulation of the sweeping gas, whilst 12 is a selectively operateddischarge for the sweeping gas. Operation of the discharge is dependenton the contamination in the circuit and supply of air is dependent onthe operation of the discharge.

[0021] The fuel cell assembly described above functions as follows:

[0022] Before putting the assembly into operation it is first necessaryto produce sweeping gas. To this end air is admitted via inlet 8. Assoon as the circuit has been filled, the supply of air is terminated orsubstantially reduced. At the same time the reforming device 30 is putinto operation, as a result of which hydrogen is introduced via thesemi-permeable membrane 5 into the sweeping gas circuit 2. Hydrogen andoxygen present in the air (and any carbon monoxide present) arecombusted in a controlled manner in burner 9. The mixture produced fromthis constitutes the sweeping gas. This sweeping gas is thencontinuously circulated by pumping. Hydrogen gas that passes into thesweeping gas circuit can now be used for the reaction in fuel cell 1. Inorder to prevent contamination of the sweeping gas circuit with gaseswhich are not consumed in the fuel cell but do accumulate, the outlet 12is operated continuously or periodically. Sweeping gas containingharmful components is discharged by this means. During operation, freshsweeping gas can also be prepared by reaction in the fuel cell byallowing restricted feed of air via inlet 8.

[0023]FIG. 2 shows the same assembly as in FIG. 1 except that the CO/CO₂feed is indicated by 18 and a methanising device is indicated by 19.Methane is now used as sweeping gas instead of air. This methane isproduced by converting CO/CO₂ to methane with the aid of hydrogen.

[0024] Here again, preparation of the sweeping gas first takes place onstarting up the cell. In this case the sweeping gas methane is preparedin the methanising device 19 as indicated above by reaction of hydrogenfrom the reforming device and CO/CO₂ which is present or is supplied. Anembodiment with which the CO/CO₂/H₂ mixture can be supplied by thereforming device is advantageous. After sufficient methane has beenprepared, the sweeping gas circuit is set in operation by pump 3 andhydrogen prepared in the reforming device can then be used on the anodeside of the fuel cell. Any carbon dioxide/carbon monoxide producedduring the process is immediately converted to methane in themethanising device 19 by the hydrogen present. Consequently there isless necessity for topping up CO/CO₂ via inlet 18 for conversion tomethane. However, discharge can be necessary because of the presence ofother components in the sweeping gas which are undesirable and must notbe allowed to accumulate.

[0025] Although the invention has been described above with reference toa preferred embodiment, it will be understood by those skilled in theart that numerous variations are possible which are obvious afterreading the above description and which fall within the scope of theappended claims.

1. Low-temperature fuel cell assembly (1) comprising a fuel cell having a sweeping gas circuit (2) for fuel gas, said sweeping gas circuit being connected to the inlet (6) and outlet (7) of the anode (11) of the fuel cell (1) and supply means (4) being provided for fuel, which is connected to said sweeping gas circuit via a physical separation device (5), means (8, 9; 18, 19) being arranged in the sweeping gas circuit for producing sweeping gas therein, characterised in that the means (8, 9; 18, 19) comprise chemical reactor means having an inlet for one component of said chemical reaction and in that the sweeping gas circuit is provided with a sweeping gas discharge (12).
 2. Low-temperature fuel cell assembly according to claim 1, wherein said means comprise an air feed (8) and a burner (9).
 3. Low-temperature fuel cell assembly according to claim 2, wherein said burner comprises a catalyst.
 4. Low-temperature fuel cell assembly according to claim 1, wherein said means comprise methanising means (19) for converting CO/CO₂ to methane in the presence of hydrogen.
 5. Low-temperature fuel cell assembly according to claim 4, wherein said means have a feed (18) for CO/CO₂.
 6. Low-temperature fuel cell assembly according to one of the preceding claims, wherein said receiving section (4) for a fuel gas comprises a semi-permeable wall (5).
 7. Method for operating a low-temperature fuel cell assembly comprising an SPFC cell and a sweeping gas circuit, in which the anode and means for introducing a fuel gas are accommodated, said fuel gas being subjected to a physical separation operation before introduction into the sweeping gas circuit and a sweeping gas being produced in the sweeping gas circuit while putting the fuel cell into operation, characterised in that the sweeping gas is produced by a chemical reaction and sweeping gas is discharged during operation of the fuel cell.
 8. Method according to claim 7, wherein the fuel gas is hydrogen.
 9. Method according to claim 8, wherein the sweeping gas is produced by introducing air and allowing the oxygen present therein to react with hydrogen.
 10. Method according to claim 8 or 9, wherein the sweeping gas is produced by providing CO/CO₂ and allowing this to react with hydrogen to give methane. 